U.S. application Ser. No. 08/010,859, now U.S. Pat. No. 5,348,604, describes a system for the application of an extrudable substance onto a substrate while the substance is coupled during extrusion with a beam of electromagnetic radiation from a laser. The extrudable substance is typically a polyamide hot melt adhesive, and the light is supplied by a 150 watt YAG laser. The substance is extruded through a small nozzle that has an inlet to a flow channel at one end and an exit at an opposite end. The light beam is coupled to the material in the flow channel by directing the beam through a window at one end and along the longitudinal axis of the nozzle. If the beam is not properly coupled into the extrudable material, the energy can be absorbed by the nozzle or the window, which substantially reduces the lifetime of those elements.